Hi Matt, I just ran some numbers on a .338 RUM. It will push a 185 grain Barnes TTSX a t 3300fps. With a 300 yard zero, you are no more than 4 inches high mid trajectory and 8.7" low at 400.... Thats from a 24" barrel. If you went with a 26" (kinda long for hunting but doable) you could push that a little... Flight time would be so short out to 400 yards that the wind wouldn't have enough time to really push the bullet that far. With a 10mph crosswind the round only drifts 10" at 400 yards. You might look into specific long range rounds like the .338 Edge, it will likely push it a little faster. That is going to be a custom route, so I don't know if that is something you would want to undertake. It also is a Wildcat round and that tends to turn some people off... but it is a fairly accepted wildcat in the long range competition circles. Here it is next to a .338 Lapua. L to R: .338 Edge, .338 Lapua, .338 Allen Express (Lapua AI), .338 Allen Magnum. Any one of these will do everything you are looking for. I would still vie for the .338 RUM though, if you can get your hands on one or build one. Like others have said, there are potential transport issues with the .338 Lapua, otherwise it would be a shoo-in, it is more commonly available and has plenty of power. Best of luck with your search!